1. Field of the Invention
This invention relates to gas turbine engines and, particularly, to the engine""s exhaust case frame in the gas path.
2. Related Art
As is generally well known, gas turbine engines are continually undergoing changes with goals of improving performance, decreasing vibration and noise for a given load rating, while reducing cost and enhancing durability, producability and repairability. To improve performance, it is typical to increase the operating temperature of the engine since increased turbine temperatures improves the engine""s efficiency and thus its performance. Of course, these increased temperatures necessitate other changes in the engine to handle the increased thermals so as to maintain structural integrity of the engine""s components.
As is known, the durability of the turbine exhaust case and its struts that are disposed in the gas path dependent on the material selected if not shielded in some way. For this reason, in new turbines it is not uncommon to shield the struts by encapsulating them with an aerodynamic fairing. The cross-section of an exhaust strut shield resembles an airplane wing with a rounded leading edge tapering off to a near knife edge at the trailing lip. These xe2x80x9cwing sectionsxe2x80x9d are set at a fixed angle, in the axial direction of the rotor shaft.
The angle of attack or impingement of the exhaust flow at the location of the strut shields is not constant. The changing angle of attack creates vortex shedding vibrations as it passes over the exhaust strut shields. Extensive cracking has been observed in the exhaust diffusers of some combustion turbine units, which is likely due to excessive vibration from the transient affects of vortex shedding off the strut shields.
Accordingly, improved performance of the engine could be obtained if vortex shedding off the strut shields were reduced and preferably eliminated. Accordingly, it is an object of this invention to provide an improved fairing shield for the exhaust struts that minimizes vortex shedding.
It is a further object of this invention to provide such an improved strut fairing design that minimizes vortex shedding at all engine speeds.
The foregoing objects are achieved employing a gas turbine engine including a turbine exhaust case having a pair of concentrically-spaced rings and a plurality of extending, elongated struts interconnected and supported by the rings. The struts are supported downstream of a last row of rotating turbine blades. A fairing encapsulates at least one of the struts. The fairing is designed in the shape of an airfoil having a substantially rounded upstream end that tapers in the downstream direction to a more narrow profile than the upstream end when viewed from the corresponding edge. The airfoil is constructed to have an upstream, stationary, annular housing which substantially surrounds the strut within the annular portion thereof. The airfoil also has a downstream tail section substantially situated downstream of the strut and pivotal about an axis parallel to the elongated dimension of the strut. Means are included for changing the angle of the tail section of the fairing about its pivot axis, preferably while the engine is operating, to minimize vortex shedding.
Preferably, a control system is provided that changes the angle of the tail section of the fairing on-line in response to changes in engine parameters such as speed or vibration to minimize the vibration in the exhaust section of the engine. The tail sections of the fairings can be controlled to move individually or preferably they can be interconnected to move as a single unit so all the tail sections of the fairings are moved at the same time and are at the same angle to the upstream stationary fairing sections. In this way, vortex shedding can be minimized and engine vibration reduced.